In many wireless communications systems, and especially in cellular communication systems, it is important to control the transmitted power of a traffic channel in order to reduce cochannel interference. Cochannel interference is generated by other transmitters assigned to the same frequency band as the desired signal. And because all users transmit traffic on the same carrier frequency in a code division multiple access (CDMA) cellular system, reducing cochannel interference in CDMA systems is especially important because it directly impacts system capacity. If the cochannel interference is reduced, the CDMA system capacity may be increased. Therefore, it is a design goal to transmit a traffic signal with only an amount of power necessary to provide acceptable signal quality at the receiver, after it passes through the channel.
In this document, a "channel" may be defined as a path or paths of communication through a medium between a transmitter and a receiver. If the medium is air and communication takes place with radio frequency (RF) signals, such a channel is typically affected by fading, which is discussed in greater detail below. A "traffic channel" may be defined as a channel that carries data, whether representing voice or other information generated by the user, which the user intends to transmit via the channel. The traffic channel may be distinguished from other channels used by the communication system that may be used to transmit timing, control, or other information supporting system operation.
Power control systems in cellular communication systems should compensate not only for signal strength variations due to the varying distance between the base station and the subscriber unit but should also attempt to compensate for channel quality fluctuations typical of a wireless channel. These fluctuations are due to the changing propagation environment between the transmitter, or base station, and the receiver, or subscriber unit, as the user moves in the service area.
There are two main groups of channel quality fluctuations: slow fading (i.e., shadowing) and fast fading. Shadowing occurs as the subscriber unit moves over uneven terrain, or travels into a propagation shadow behind a building or a hill or other obstacle much larger than the wave length of the frequency of the wireless channel. Fast fading occurs when electromagnetic waves transmitted from the transmitter follow multiple paths on the way from the transmitter to the receiver. The different paths have different delays and interfere at the antenna of the receiver. If two paths have the same propagation attenuation and their delay differs in an odd number of half-wave lengths (half-periods), the two waves may cancel each other at the antenna completely. If the delay is an even multiple of the half-wave lengths, the two waves may constructively add, resulting in a signal of double amplitude. The fluctuation of the channel gain between these two extremes is called fading.
Since the scattering and reflecting surfaces in a service area are randomly distributed, the amplitude of the resulting signal is also a random variable. The amplitude of fading is usually described by a Rayleigh, Rice, or Nakagami distributed random variable.
Since the subscriber unit may move at the velocity of a moving car or even of a fast train, the rate of channel fluctuations may be quite high and the power control has to react very quickly in order to compensate for the rapid fluctuations. The rate of fading is usually expressed in terms of Doppler frequency.
Existing power control systems used in CDMA cellular systems that operate according to J-STD-008, published by the Joint Technical Committee on Wireless Access, use the measurement and reporting of cyclic redundancy check (CRC) errors at the subscriber unit to control the power of the traffic channel at the base unit. This method of power control in response to CRC errors is used to implement a slow "ramping" power control scheme. The "ramping" occurs because the traffic channel power is increased by a relatively large amount when the subscriber unit reports a CRC error. After the large power increase, which often eliminates the CRC errors for some subsequent period, the power is reduced by a relatively small amount for each subsequent frame transmitted. Eventually, the power is reduced to a point where another CRC error occurs, and the power is once again increased by a relatively large amount. If channel quality remains constant, a graph of power transmitted in the traffic channel resembles a saw tooth, with large power increases followed by a series of small power decreases.
One problem with this method of power control is the delay encountered between the degradation of channel quality and the request for a power increase and the subsequent actual increase in power. The delay in requesting a power increase is caused by waiting for a frame to be received, and then waiting for frame decoding and the detection of a cyclic redundancy check error. Once the CRC error is detected, it must be reported to the base station, and the base station must respond by increasing traffic channel power. In current CDMA systems, it takes 20 milliseconds (mS) to receive a frame. Thus, the rate at which CRC reports or power control commands are sent to the transmitter is 50 Hz. This delay in the power control loop periodically causes the base to transmit too much power on the traffic channel, such as when a relatively large increase in power is requested and granted just as the channel quality has reached a minimum and starts to improve. If the traffic channel has too much power, cochannel interference increases and system capacity decreases.
With reference now to FIG. 1, there is depicted a set of graphs that illustrate a prior art method of controlling traffic channel transmit power, and the relationships between channel quality, traffic channel transmit power, and received frame quality. As illustrated, FIG. 1 includes channel quality graph 20, traffic channel transmit power graph 22, and received frame quality graph 24. The x-axis of each of these graphs shows frame numbers, which correspond to 20 mS units of time. The y-axis of channel quality graph 20 is a signal-to-noise ratio, expressed as E.sub.b /N.sub.t. The y-axis of traffic channel transmit power graph 22 is power in dB relative to the maximum power available at the transmitter. The y-axis of received frame quality graph 24 is also signal-to-noise ratio, E.sub.b /N.sub.t. A 20 mS frame duration is shown at reference numeral 26.
As shown in graph 20 at time 28, channel quality has begun to decline. In response to this channel quality decline, the received frame quality is also declining, as shown at time 28. Eventually, received frame quality becomes low enough to cause a CRC error. The error is reported to the base station, and the base station responds by increasing the power by a relatively large amount, as shown in graph 22.
It should be noted here that the delay between detecting a need for an increase in transmit power and the actual increase in transmit power is not shown in graphs 20-24. Thus, these graphs show an increase in transmit power at the same time the need for the increase in power is detected. The delay between detection and power increase actually experienced in the prior art method increases the likelihood that transmitted power will exceed the power necessary for a desired frame quality at the subscriber unit.
Problems with the prior art are further illustrated at time 30, where a relatively large increase in power is requested and granted just as channel quality reaches a minimum and starts to increase. Increases in transmit power when the channel quality is increasing causes an excess in transmitted power. Following time 30, the power is decreased by relatively small amounts, thereby forming the sawtooth wave form.
In order to improve control of transmitted power, it has been proposed that power control commands be sent from the receiver to the transmitter at a rate greater than 50 Hz. By sending power control commands from the receiver more often, the amount of power change requested can be smaller, and the system can respond more quickly to changes in channel quality. This reduces the likelihood of a request for an increase in power for the next 20 mS frame just as the channel quality begins to increase.
However, new problems arise in systems that attempt to control power faster than the 50 Hz frame rate. For example, the power control metric in an IS-95 CDMA system is the detection of a CRC error. This is a problem because CRC errors cannot be detected based upon the receipt of a partial frame. If power is to be controlled at a frequency greater than 50 Hz, then a new metric for determining whether or not to increase or decrease power is needed. This new metric should be measured at the higher rate at which power control commands are sent to the transmitter. Furthermore, it is difficult to measure any metric related to the traffic channel because the transmission rate of the traffic channel is not known until an entire 20 mS frame is received and decoded.
In a CDMA system which decodes frames to check for CRC errors, frames may be decoded using a Viterbi decoder, which is a maximum likelihood sequence estimator. Before CDMA signal samples are input into the Viterbi decoder, the samples are calibrated, or weighted, according to a complex weighting coefficient derived from the amplitude and phase of the pilot signal. The purpose of the complex weighting coefficient is to compensate the symbols for the phase rotations introduced by the channel. Furthermore, the weighting adjusts the gain of the symbols according to the gain variations in the channel.
Because, in a prior art IS-95 CDMA system, the transmitted traffic channel power at the transmitter does not change over the duration of a 20 mS frame, the complex weighting coefficient may be calculated by analyzing the phase and gain of only the pilot signal.
With reference now to FIG. 2, there is depicted a prior art circuit for generating a complex weighting coefficient for use in a maximum likelihood decoder. As illustrated, traffic channel demodulator 31 includes antenna 32, which receives radio frequency CDMA signals. After receiving the signals, the signals are down converted by removing the carrier (not shown) and coupled to despreaders 34 and 36. Despreader 34 despreads a pilot signal using Walsh code 0. Despreader 36 despreads the traffic channel using a traffic channel Walsh code assigned by the CDMA system.
After despreading the pilot signal, the despread output is filtered, as shown by finite impulse response filter 38. Filter 38 reduces noise that remains following the despreading operation.
The output of filter 38 is then coupled to complex conjugate function circuit 40, which changes the sign of the imaginary part of the pilot signal.
The output of complex conjugate function circuit 40 is the complex weighting coefficient, .varies.e.sup.j.theta., which is then multiplied by the despread traffic channel signal from despreader 36, as shown at multiplier 42. Thus, the output of multiplier 42 is a despread traffic channel signal that has been compensated for the gain and phase effects of the channel.
The output of multiplier 42 may contain some noise having an imaginary component, which is removed by circuit 44. The output of traffic channel demodulator 31 is a demodulated traffic channel signal comprising traffic channel symbols. The demodulated traffic channel symbols are then coupled to deinterleaver and decoder 46.
Deinterleaver and decoder 46, which decodes a frame of symbols that are represented by real numbers, is coupled to the output of circuit 44 for decoding frames. The decoder may be implemented with a Viterbi decoder.
In some instances, the demodulator shown in FIG. 2 does not generate the best complex weighting coefficients for a system that varies the transmit power of the traffic channel. This is because complex weighting coefficients generated from the pilot signal alone do not reflect a recent change in traffic channel power--the quantity for which a power control system exists to control. That is, the controller needs an accurate and immediate measurement of the quantity it is controlling.
Therefore, it should be apparent that there is a need for an improved method and system for controlling traffic channel transmit power in a wireless communications system, and especially in a CDMA cellular system. The improved power control method and system should minimize excess power transmitted in the traffic channel, generate power control commands at a rate greater than known power control systems, and use a metric that can be measured several times during the reception of a frame, wherein each measurement can be made after receiving only a portion of a frame.